[v26.1.x] Port chunked_vector, chunked_hash_map, and use it in prometheus-impl.hh#288
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WillemKauf merged 7 commits intoJul 1, 2026
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Pull request overview
This PR ports Redpanda’s chunked_vector and chunked_hash_map into Seastar, wires the new hash map into the Prometheus label aggregation path, and updates the build/dependency plumbing (CMake, cooking ingredients, pkg-config, and install scripts) to support the new containers.
Changes:
- Add new core containers:
include/seastar/core/chunked_vector.hhandinclude/seastar/core/chunked_hash_map.hh. - Switch
src/core/prometheus-impl.hhlabel aggregation map fromstd::unordered_maptochunked_hash_map. - Add unit tests for both containers and add Abseil + unordered_dense as dependencies (CMake/cooking/pkg-config/install scripts).
Reviewed changes
Copilot reviewed 11 out of 11 changed files in this pull request and generated 9 comments.
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| File | Description |
|---|---|
| tests/unit/CMakeLists.txt | Registers new unit tests for chunked containers. |
| tests/unit/chunked_vector_test.cc | Adds extensive iterator/behavior tests for chunked_vector. |
| tests/unit/chunked_hash_map_test.cc | Adds basic compile/behavior tests for chunked_hash_map and from_range. |
| src/core/prometheus-impl.hh | Uses chunked_hash_map for metric aggregation by labels. |
| pkgconfig/seastar.pc.in | Adds cflags for unordered_dense and Requires for Abseil hash. |
| install-dependencies.sh | Adds distro packages for Abseil. |
| include/seastar/core/chunked_vector.hh | Introduces chunked_vector implementation and iterators. |
| include/seastar/core/chunked_hash_map.hh | Introduces chunked_hash_map/set, helpers, and fmt integration. |
| cooking_recipe.cmake | Adds cooking ingredients for unordered_dense and Abseil. |
| CMakeLists.txt | Installs new headers and links against unordered_dense and Abseil hash target. |
| cmake/SeastarDependencies.cmake | Finds unordered_dense and Abseil via CMake. |
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| } | ||
| ~chunked_vector() noexcept = default; | ||
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| chunked_vector copy() const noexcept { return *this; } |
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| void reserve(size_t new_cap) { | ||
| static constexpr size_t elems_per_frag = calc_elems_per_frag(); | ||
| if (new_cap > _capacity) { | ||
| if (_frags.empty()) { | ||
| auto& frag = _frags.emplace_back(); | ||
| frag.reserve(std::min(elems_per_frag, new_cap)); | ||
| _capacity = frag.capacity(); | ||
| } else if (_frags.size() == 1) { | ||
| auto& frag = _frags.front(); | ||
| frag.reserve(std::min(elems_per_frag, new_cap)); | ||
| _capacity = frag.capacity(); | ||
| } | ||
| // We only reserve the first fragment as all fragments after the | ||
| // first are allocated at the maximum size, so we don't save | ||
| // anything in terms of reallocs after fully allocating the | ||
| // first fragment. In addition, due to cache locality, it's | ||
| // better to delay the allocations of those other fragments | ||
| // until they're going to be used. | ||
| } | ||
| update_generation(); | ||
| } |
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| iter& operator+=(ssize_t n) { | ||
| check_generation(); | ||
| _index += n; | ||
| return *this; | ||
| } | ||
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| iter& operator-=(ssize_t n) { | ||
| check_generation(); | ||
| _index -= n; | ||
| return *this; | ||
| } |
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| friend ssize_t operator-(const iter& a, const iter& b) { | ||
| return a._index - b._index; | ||
| } |
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| pointer operator->() const { | ||
| check_generation(); | ||
| return &_vec->operator[](_index); | ||
| } |
Author
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no one is using chunked_vector<bool> anyways 😛
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| // Calling shrink to fix then modifying the container could result | ||
| // in allocations that overshoot our max_frag_bytes, except when | ||
| // we're managing the dynamic size of the first fragment. |
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| if (f.capacity() > std::decay_t<decltype(v)>::max_frag_bytes()) { | ||
| return fmt::format( | ||
| "fragment {} capacity over max_frag_bytes ({})", i, calc_cap); | ||
| } |
Author
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oops. thankfully this is test code!
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| /** | ||
| * Proxy that applies a consistency check before deference | ||
| */ |
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| return m.bucket_count() | ||
| * sizeof(typename chunked_hash_map<K, V>::bucket_type) | ||
| + m.values().capacity() * sizeof(m.values()[0]); |
WillemKauf
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Wire ankerl::unordered_dense into the build as a required dependency: find_package in SeastarDependencies, link unordered_dense::unordered_dense PUBLIC, a cooking recipe pinned to the commit Redpanda consumes (v4.4.0), and propagate its include flags to consumers via the pkg-config file. This is a prerequisite for the upcoming chunked_hash_map container.
Wire abseil into the build as a required dependency for chunked_hash_map's absl::Hash support: find_package(absl CONFIG), link absl::hash PUBLIC, a cooking recipe pinned to the same LTS release Redpanda consumes (20250814.1), the absl_hash pkg-config requirement for consumers, and the abseil dev packages for the distros that reliably carry them. This is a prerequisite for the upcoming chunked_hash_map container.
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Port chunked_vector from Redpanda: a random-access vector whose storage is split across fixed-size fragments instead of one contiguous block, so it grows without large reallocations and keeps element addresses stable across growth. Adapted to Seastar conventions (seastar namespace, SEASTAR_ASSERT, .hh header) with a Boost.Test port of the test suite.
Port chunked_hash_map/chunked_hash_set from Redpanda: ankerl's segmented hash map/set backed by chunked_vector storage, suited to maps that grow large (scaling with partitions or topics) without large contiguous allocations. Hashing dispatches to absl::Hash when a type provides AbslHashValue, otherwise to unordered_dense's hash. Adapted to the seastar namespace, with a Boost.Test port of the test suite.
Replace the std::unordered_map backing metric_aggregate_by_labels with chunked_hash_map so per-label aggregation storage grows without large contiguous allocations when exporting many metric series.
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@StephanDollberg are you happy with how these deps are fetched? vendoring was an option (for unordered_dense at least) but the CMake changes feel fine |
Member
I think that's fine. cmake is really only for, hacking on seastar in isolation so that looks fine. |
StephanDollberg
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StephanDollberg
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ankerl::unordered_densedepabsldepchunked_vector.hchunked_hash_map.hchunked_hash_mapinprometheus-impl.hhto avoid oversized allocations in the metrics aggregation pathFor v26.2.x patch, see: #289